1. A Field of the Invention
The present invention relates, in general, to a vibration apparatuses and more particularly to a vibration apparatus for a variable amplitude type vibration table and which generates vertical vibration for the use of, for example, a concrete block molding machine.
2. Description of Prior Art
Many types of vibration apparatus have been known in the art which have vibration tables, vibration shafts having eccentric weights for vibrating the vibratory table. An example of such vibration apparatsu as described above is a vibration table for a concrete block molding machine which is disclosed in Japanese Patent Publication (unexamined) No. 2-80203 in which a material for concrete blocks is supplied into a mold form and a vibration table is provided for receiving the mold form. At a lower position of the vibration table, two vibration axial bodies of a dual axial structure having weights are provided so that the position of the weights can be directly changed by a hydraulic or an oil pressure device. In this conventional device, the position of the weights is moved or changed by a sliding movement of the hydraulic piston so that an amplitude of the vibratory movement is varied.
However, in the conventional vibration mechanism of a dual-axis structure which has a single axis for serving as an inner cylinder and an outer cylinder, the entire structure of the mechanism is so complex that it is quite difficult to provide unbalanced positions of vibration by means of a phase difference and to control or regulate the position of the weights. Thus, it has been difficult to incorporate and assemble the vibration mechanism into the apparatus, and substantial labor is required to maintain of the apparatus.
It has been attempted to provide two vibratory shafts of a dual axis structure are provided into a central portion of the vibration table so that the inner cylinder serves as an unbalanced shaft having a weight. The unbalanced shaft is provided with a helical cam groove and a control pin fitted to the helical cam groove so that the control pin is moved longitudinally to shift or change the phase of the weight to thereby obtain a predetermined vibration. This type of vibration mechanism, however, is still disadvantageous in that a sliding movement of the control pin relative to the helical cam groove is not as smooth as desired and it is likely that an undesirable displacement is generated, with the result that it has been difficult to obtain a predetermined, constant vibration to an entire area of the vibration table.